1. Field of the Invention
The present invention relates to ignition control for gas-fired, forced combustion air heating appliances.
2. Description of the Prior Art
To obtain higher efficiency, many gas-fired furnaces use induced-draft combustion air blowers and electronic fuel ignition. A standard approach for this type of heating appliance has been to use the closure of the thermostat heat demand contacts to power a combustion air blower relay, which turns on the combustion air blower. The ignition control is then powered through a combustion air sensor (the "combustion air proving pressure switch"), which closes when the combustion air blower is delivering combustion air.
An important consideration for any gas-fired heating appliance is safety in the case of a failure of one or more of the components of the appliance and its ignition control system. For example, Underwriters Laboratory requirements for failure modes and effects analysis (FMEA) require that all single component failures either cause a safe shutdown of the appliance or, if undetected, the ignition control must continue to operate safely. If the ignition control continues to operate after a first order component failure, then for all combinations of second order failures, it must either continue to operate safely, or shutdown safely.
Two potential problems exist with the standard control system for an induced-draft forced combustion air furnace. First, most combustion air proving pressure switches have a single pole normally open (SPNO) contact configuration. A welded contact in the pressure switch is not detectable. This by itself may not pose a serious problem, since there are various other steps which must take place before ignition. However, if the combustion air blower also were to fail when the combustion air proving pressure switch has its normally open contacts welded closed, the ignition control would attempt ignition without a combustion air supply. This is a potentially unsafe condition.
Second, in those ignition control systems which have only a single relay controlling the gas valve, the welding shut of the normally open contacts of the gas valve control relay leaves the ignition control with no way to shut off the gas valve. While the drive circuit which controls the gas valve control relay may be failsafe, the welding of the relay contacts leaves the control of the gas valve solely to the operation of the thermostat heat demand contacts.
At the present time, these two potential problems are addressed by requiring that the pressure switch and the gas valve relay pass certain life test requirements. This, however, addresses the two potential problems in an indirect way, and does not provide for safe operation or shutdown in the event that one of these two components does, in fact, fail.